For a general description of installations for transmission by means of high-voltage direct current, reference is made to J. Arillaga: High Voltage Direct Current Transmission, London 1988.
Examples of the use, design and connection of active filters in connection with such installations are given in W Zhang, G Asplund, A .ANG.berg, U Jonsson, O Loof: Active d. c. filter for HVDC system, ABB Review 1995, Nos. 6-7, pages 17-21, in L Gyugui, E. C. Sstrycula: Active AC Power Filters, IAS 76 Annual, pages 529-535, and in IEEE publication 89 WM 123-1 PWRD (IEEE/PES 1989 Winter Meeting, New York 1989): Cheuksum Wong, Ned Mohan, Selwyn E. Wright and Karl N. Mortensen: Feasibility Study of AC- and DC-Side Active Filters for HVDC Converter Terminals.
U.S. Pat. No. 5,296,764 describes a method for reduction of disturbances on an outgoing line from an electric power network by means of an active filter, the control order of which is formed in dependence on a control signal from a controlling device. This control signal is formed in dependence on the deviation of a current or a voltage, sensed in the power network, from a desired value, and is forwarded to the control order with such signs that the disturbances are reduced. According to the described method, the control signal is formed from the controlling device as a sequence of signal values, repeated with a certain periodicity corresponding to the periodicity for a source of disturbance in the power network. The feedback circuit comprises a filter which, at least within the frequency range of interest to the application, makes possible a linearization of the transfer function from the control signal of the controlling device to the deviation of the sensed current or voltage from the desired value, a compensation for non-minimum phase properties in the transfer function mentioned being included therein. By utilizing the periodicity of the disturbance source, a control circuit with negative feedback may, with good stability and damping, be implemented also for those applications where, for example, a long line is connected to the power network. The effort required for setting of parameters in the filter which compensates for the above-mentioned transfer function is, however, demanding and increases as a better approximation of the dynamic properties of the power network within the frequency range in question is aimed at. Further, switching operations in the power network lead to its transfer function being changed, and to this is to be added the fact that the frequency of the disturbance source is usually not completely stable.
An article in a periodical by C Tuttas: Anwendung aktiver Saugkreise in elektrischen Energieversorgungsnetzen, etzArchiv 1987, H.4, pages 93-100, describes a three-phase active filter for connection to an ac network for damping of resonances and certain definite harmonic currents in the network. A controlling unit for control of a filter current of a certain frequency .omega..sub.f comprises a comparison circuit for forming a control deviation between a sensed filter current and a reference value therefor. The control deviation, which is an ac signal of the form .DELTA.I*sin(.omega..sub.f t+.phi.) is converted, by multiplication by a signal of the form sin (.omega..sub.f t) and by a signal of the form cos (.omega..sub.f t) as well as by a current mean-value formation of the respective signals, obtained by the multiplications, into two components .DELTA.I.sub.sin =.DELTA.I*cos .phi. and .DELTA.I.sub.cos =.DELTA.I*sin .phi., respectively. These components, which thus are of the frequency zero, together contain information about the amplitude and phase position of the control deviation. Each one of these components is supplied to a respective controller with proportional-integrating characteristic. The output signals from the respective controller are multiplied by respectively one signal of the form sin (.omega..sub.f t) and one signal of the form cos (.omega..sub.f t), and are then added to form a control signal of the frequency f for the active filter. This control signal is supplied to which is shown in the article as a block designated `Matrix` comprised in the controlling unit. The purpose of this block is stated to be to decouple and adapt the controller to the control distance, by which is probably meant an adaptation to the transfer function from the control signal to the sensed filter current.
This `Matrix` is further discussed in an article in a periodical by C Tuttas: Regelung von Blindleistungskompensatoren mit aktiven Filtern. Automatisierungstechnik 1986, H.8, pages 302-310, from which it seems to be clear that the parameters in this `Matrix` are calculated on the basis of externally known data about the ac network and component data for the filter. The accuracy in the adaptation to the above-mentioned transfer function is thus dependent on how well known the data are on which the calculation is based and, as mentioned above, switchings in the power network lead to its transfer function being changed and to this is to be added the fact that the frequency of the disturbance source is usually not completely stable.